278 NORMAL HISTOLOGY. 



efferent fibres exist. Such fibres, connecting the motor cells of the cerebral 

 cortex with the motor root-cells of the spinal nerves, are condensed into two 

 chief strands, the anterior and lateral pyramidal tracts. The latter 

 (fasciculus cercbro-spinalis lateralis} occupies an oval area between the 

 lateral aspect of the posterior horn and the direct cerebellar tract. The 

 component fibres are the axones of the motor cortical neurones, which have 

 descended from the white core of the cerebrum, through the ventral part of 

 the brain-stem (cerebral peduncle, pons and medulla), to the lower part of 

 the medulla and thence, after crossing in the pyramidal decussation (page 

 283), into the lateral column of the cord. On reaching the level correspond- 

 ing to the cord-segment to be influenced, the cerebro-spinal fibres bend 

 inwards, enter the gray matter, and end around the motor root-cells jn 

 arborizations. The fibres of the anterior or direct pyramidal tract 

 (fascicuhis cerebro-spinalis anterior) correspond in origin and course with 

 those of the lateral tract until they reach the lower part of the medulla, 

 where, instead of crossing in the pyramidal decussation, they continue into 

 the anterior column and descend within a narrow zone along the anterior 

 median fissure of the cord. Although not sharing the decussation in the 

 medulla, practically all of these fibres cross somewhere, by way of the anterior 



white commissure, and pass at the 

 appropriate level into the anterior 

 cornu of the opposite side, to end in 

 arborizations around the radicular 

 cells. In no case does a motor im- 

 pulse pass directly from the cerebral 

 cortex to the muscle fibre, since 

 always at least two links the corti- 

 cal and the spinal neurone are re- 

 quired to complete the chain. 



The existence of direct paths 

 from the cerebellar cells to those of 

 the spinal cord is uncertain, the im- 

 pulses from the cerebellum being 

 usually carried by the axones of the 

 FIG. 3 25-Section of cord through lower part of cerebellar neurones to cells within 

 s U Tner r ) region ' X 6 ' (Preparalion by Professor the brain-stem (red, vestibular and 



olivary nuclei) and thence by secon- 

 dary axones (rubro-, vestibule-, and olivo-spinal fibres) through the antero- 

 lateral columns to the gray matter of the cord. 



Although the tracts above described include the more definite and 

 evident of the paths of conduction, a glance at Fig. 321 shows that a con- 

 siderable part of the anterior and lateral columns of the cord is still un- 

 accounted for. Concerning this extensive area, to which the name antero- 

 lateral ground-bundle is conveniently applied, much uncertainty exists; 

 suffice it to point out, that within its complex are many fibres of importance, 

 some of which connect the cord with distant parts of the brain, while others 

 serve as links binding together different levels of the cord itself. These last, 

 the intersegmental association fibres, are for the most part short, 

 passing as the axones of the posterior horn cells into the adjacent white 

 matter and, after a limited course, bending inwards to enter once more the 

 gray matter and end around the nerve-cells at some different level. The 

 shorter association tracts lie close to the gray matter, while the longer ones 

 run within the more peripheral part of the antero-lateral ground-bundle. 



